Re: what makes diffrent things rare?

From http://www.derm
.med.ed.ac.uk/teaching/redhairgen.htm
-------------------------------
by J.L. Rees   Red Hair Genetics
...
Several years ago, I and colleagues, discovered that the melanocortin 
1 receptor, a protein encoded by a gene previously discovered in mice, 
was responsible for the production of red hair in humans.  Everybody 
has two copies of this gene but there are slight changes in the gene 
that are very common in European populations.  If you have one of 
about four or five common changes in this gene and, one of these 
changes are found on both of your chromosomes, then you are likely to 
have red hair.  A little bit of basic genetics; you have two copies 
of every gene, you inherit one from your mother and one from your 
father.  If both of these genes are different, with respect to the 
changes that might lead to red hair, then you will have red hair.  
If however you only have one change, you have an increased chance 
of having red hair but it is not certain that you will have red 
hair. 

Such a type of inheritance is described to by geneticists as an 
autosomal mode of inheritance.  This means, in practice, that both 
your parents may not have red hair, but both could be carriers for 
the gene for red hair.  If this was the case, perhaps one in four 
of their children might have red hair.  If one of the parents has 
bright red hair, and therefore carries two of the changes (one on each 
of their chromosomes), and the other parent is a carrier, then perhaps 
50% of their children might have red hair.  It is this aspect of 
genetics, and the mode of inheritance, that explains why hair colour 
might skip generations.

There are different sorts of red hair.  Some people seem to have what 
we call “strawberry blonde”, some bright red and some auburn.  As far 
as we know, the genetics underlying these differences are fairly 
similar, in that changes in the gene referred to above, seem to 
be important for all sorts of red hair.  However, if you have bright 
red hair it seems you are much more likely to carry two different 
copies of the gene than if you are a strawberry blonde.  We are, 
however, not completely certain about some of the details in 
this particular aspect of the work.  
...


From http://www.howstuffwork
s.com/question593.htm 
--------------------------------
U.S. Blood-type Distribution
O+  38 percent of population 
A+  34 percent of population 
B+   9 percent of population 
O-   7 percent of population 
A-   6 percent of population 
AB+  3 percent of population 
B-   2 percent of population 
AB-  1 percent of population 

-----------------------------------------------------------
   Notice in the www page about red hair, that the author is
not talking about a "red hair gene".  He (or she) is talking
about differences in one gene.  This is most often the case
in genetics, that there is not a "red hair gene" and a "blonde
hair gene" and a "brown hair gene" but instead there are a few
genes that control something like hair color, and each of those
genes can have various different forms (usually just single point
mutations when we are talking about humans) which are called
"alleles".  

   There are two basic reasons why an allele can be rare.  One is that
it is a new one that has not had time to spread through a population 
yet.  The other is that the allele is detrimental (or less beneficial 
than the alternative alleles) so it is being selected out of the 
population.  Most alleles for most genes are relatively neutral or
can be beneficial in some circumstances and detrimental in others.  
The result is that we see an increase in diversity over time, as more
alleles accumulate in a population.  That diversity then tends to
create a stronger species as a whole, because the diversity provides
many more chances that one of the diverse types will be strong and
fit for surviving a change.  For a simple example, people who can
gain weight on very little food are best fit for surviving famine 
but may be less fit for survival when too much food is available 
(they have risks for heart disease and diabetes).  People who stay slim
no matter how much food is available are less fit for surviving famine
but slightly more fit for surviving in abundance.  A pouplation of
diverse people, some slim and some not, has the best chance overall
to survive in cycles of abundance and famine.

     In many or most cases, the traits we observe are not linked to 
the genes (or alleles) and their benefit or detriment to survival in
a simple way.  For example the alleles that lead to red hair may have
some benefit to people who live in northern climates and wear a lot
of clothes.  Sunlight is needed to produce vitamin D in humans, and
the alleles for red hair are either directly involved or linked to
alleles for skin color.  Dark skin needs more sunlight to produce 
vitamin D, light skin needs less sunlight because there is less pigment
blocking the sunlight.  So light skin has a slight selective advantage 
in the far north (and south, but there is less human-inhabited land mass
down there).  Dark skin has a slight selective advantage near the
equator and at high altitudes (because too much sun causes sunburn,
skin cancer and other problems).  The human race as a whole is best
suited for survival if we have individuals that differ in many traits.